1. Field of the Invention
This invention relates to a process for positioning an assembly tool, such as, for example, a drilling or riveting tool located on the end of an articulated arm, making it possible especially to prevent sliding of the tool, and if necessary to correct the angle of incidence of the tool so as to position it perpendicular to the surface to be machined, the process of the invention being more particularly designed for the field of aeronautics. It likewise relates to a device for implementation of said process.
2. Description of the Related Art
In the field of aeronautics, many parts of the fuselage and of the wings of an aircraft are assembled by riveting. This assembly can call for stages consisting of positioning the pieces to be assembled relative to one another, drilling the two pieces, checking the holes if necessary, applying a sealing compound, installing a fastener such as a rivet, and finally checking the resulting assembly. These different stages are repeated for all of the holes that are placed according to a given mapping.
So that the components are correctly assembled, the mapping of the holes must be observed, and preferably the holes and/or the rivets must be arranged perpendicular to the surface.
To carry out this assembly, a first approach consists in using a special machine with Cartesian movements. This type of machine comprises a frame with two side rails along which a crosspiece can be relocated, along which crosspiece a telescoping arm can be relocated, on the end of which there can be a tool. The side rails, the crosspiece and the telescoping arm comprise the three axes of relocation, movements of rotation being possible at the level of the end of the arm.
The advantage of this type of machine is its having a rigid structure that allows application of relatively strong forces via the tool, especially during the drilling stage, while avoiding the sliding of the tool in order to observe the mapping of the holes.
However, this type of machine is subject to the major drawback of being stationary. Thus, it is necessary to move the elements to be assembled underneath this structure; this can entail a risk of damage for said elements. Moreover, this machine can only accept elements to be assembled that do not exceed a certain template function of the dimensions of the structure. Consequently, this type of machine is normally reserved for items of small overall dimensions.
Actually, even if it is possible to imagine a structure of large dimensions, the latter due to these dimensions would have dimensional variances such as would not allow the mapping of the holes to be followed. Moreover, the cost of such a structure would be prohibitive.
To mitigate these drawbacks, the assembly operation can be implemented by using an articulated arm that comprises on its free end a tool holder that is hereinafter called an effector.
The articulated arms can move easily around the structure of an aircraft during assembly; this allows assembly of elements of large dimensions and makes it possible to avoid moving them, in contrast to machines with Cartesian movements. According to another advantage, the articulated arms are products that are marketed in large numbers, such that this approach costs less than special machines with Cartesian movements.
In the known manner, an articulated arm generally includes a pivoting base on which a first segment is attached that can pivot along a first horizontal axis of rotation, a second segment being articulated along a second horizontal axis of rotation relative to the free end of the first segment. The free end of the second segment generally comprises a third articulation, or even a fourth articulation according to two essentially perpendicular axes of rotation.
These different articulations make it possible to move the effector in a space of three dimensions X, Y, Z and to orient it along three axes of rotation Rx, Ry, Rz.
The effector generally comprises a rigid frame with, to the front, a surface that can be pressed against the element to be machined or to be assembled, and to the rear, means of coupling to the free end of the articulated arm, said frame supporting a tool or a drum that integrates several tools as well as means for gripping, moving and operating said tool or tools and, if necessary, other accessories.
During drilling or assembly, the force applied by the tool is transmitted to the effector by the articulated arm. Means of measuring this force are generally provided at the level of coupling between the effector and the arm.
The primary drawback of this approach lies in the fact that the force that can be transmitted by the articulated arm is limited. Actually, even if the effector is rigid, the articulations of the articulated arm have a tendency to be deformed if the force exceeds a certain threshold, such that the tool tends to slide at the level of the surface and the mapping of the holes can no longer be observed. In the same way, if the force exceeds a certain threshold, the tool tends to no longer be perpendicular to the surface, which can ruin the quality of the hole or of the assembly.